This invention relates to a method of forming a patterned coating film of a metal compound such as oxide or nitride on a substrate surface by a sol-gel method using a metal alkoxide sol, the method incorporating a selective etching process.
For various purposes including electronics, optics and ornaments, currently it is a very important process to form a think film of oxide, nitride or semiconductive material on a substrate surface in a desired pattern, and various methods are known for accomplishment of patterned coating. For example, in a PVD method such as vacuum evaporation or sputtering it is often to provide the substrate with masking in a desired pattern.
Also it is prevailing to first form a coating film over the entire area of the substrate surface and then remove the film in the unnecessary areas by an etching treatment. For example, in the case of a film of SnO.sub.2 or In.sub.2 O.sub.3 - SnO.sub.2 the etching treatment includes overlaying the film with a resist layer, patterning the resist layer by photolithography and then etching the oxide film masked with the resist by using a suitable etchant such as an aqueous solution of an acid or a metal chloride or hydrogen gas generated by reaction of a metal powder with an acid. For some applications patterning of a coating film by laser beam scanning is also known.
However, these conventional patterning methods are not always very effective for patterning of thin films of some oxides such as SiO.sub.2 , TiO.sub.2, ZrO.sub.2 and A1.sub.2 O.sub.3 or corrosion resistant nitrides such as TiN and SiN, and in some methods the use of a harmful and corrosive etchant such as hydrofluoric acid offers a serious problem.
Recently growing interest has been shown in the applications of a so-called sol-gel method to forming of patterned coating films. In a sol-gel method a coating film of a metal alkoxide sol is gelled by hydrolysis on a substrate surface, and then the gel film is baked to accomplish pyrolysis. There are some proposals as to the ethching method to be combined with a sol-gel method. For example, JP-A 59-6541 proposes first hardening a metal alkoxide gel film on a substrate surfadce by heating, overlaying the hardened film with a resist film, patterning the resist film by lithography to use it as a mask and etching the hardened film in the unnecessary areas with, e.g, a reactive gas. JP-A 60-1711 proposes first hardening a silicon alkoxide gel film on a substrate surface by a mild heat treatment, forming a patterned film of an alkali resistant resist on the hardened film by screen printing and etching the hardened film in the unnecessary areas with an alkali solution.
As represented by the above Japanese publications, it is a common thought to harden an alkoxide gel film by a heat treatment in advance of making an etching treatment using a patterned resist film as masking. However, the hardening affords the coating film some resistance to corrosion, whereby the subsequent etching becomes rather low in efficiency. Therefore it is necessary to continue the etching operation for a long time and/or use a very corrosive acid or alkali as etchant, and such intense etching offers difficulty to precise patterning because even in the masked areas the hardened film is liable to undergo erosion to some extent.